Apparatus and method for controlling thickness of molten metal coating by a moving magnetic field



June 30, 1970 J. w. HALLEY 3,518,109

APPARATUS AND METHOD FOR CONTROLLING THICKNESS OF MOLTEN METAL COATING BY A MOVING MAGNETIC FIELD Filed Jan. 15, 1968 5 Sheets-Sheet 1 22 @L/ 21 HH 2:233:22: 222222222 y 7 6 z a 2 E I I g 8 J 2 IIIIIIIIIIIIII June 30, 1970 J. w. HALLEY 3,518,109

APPARATUS AND METHOD FOR CONTROLLING THICKNE 01- MOLTEN METAL COATING BY A MOVING MAGNETIC 1 E D 1-; Shoots-Sheet :1

Filed Jan. 15, 1968 DIRECTION OF THE MOVING FIELD J. W. HALLEY APPARATUS AND METHOD FOR CONTROLLING THICKNESS OF MOLTEN June 30, 1970 METAL COATING BY A MOVING MAGNETIC FIELD 5 Sheets-Sheet 5 Filed Jan. 15, 1968 58 A-4 A-9 B-H CONTINUOUSLY VARIABLE AUTO- TRANSFORMER United States Patent 3,518,109 I APPARATUS AND METHOD FOR CONTROLLING THICKNESS OF MOLTEN METAL COATING BYv A MOVING MAGNETIC FIELD James W. Halley, Dune Acres, Ind., assignor to Inland Steel Company, Chicago, Ill., a corporation of Delaware Filed Jan. 15, 1968, Ser. No, 697,922 Int. Cl. C23c N02 US. Cl. 11793.2 9 Claims ABSTRACT OF THE DISCLOSURE This disclosure deals with apparatus for controlling the thickness of a metal coating applied to a strip by a hot dip coating process. The coating control apparatus includes means for setting up a strong magnetic field, and the strip, with the metal coating still in its molten state, is passed through the magnetic field. The resulting interaction between the magnetic field and the metal coating results in a force on the coating which wipes the coating to a desired thickness.

Metal coated strips, such as galvanized steel and aluminum coated steel, are usually manufactured by the hot dip process wherein the strip is drawn through a pot of molten coating metal. The coating thickness has usually been controlled in the past by a pair of grooved coating rolls between which the strip passes, a desired coating thickness being obtained by adjusting the distance between the rolls and other well known operating variables. The use of coating rolls for this purpose is, however, disadvantageous because the rolls require continuous maintenance, and coating thicknesses less than .001 inch are difficult to produce and control with such rolls. High pressure jets of steam or other gas have also been used either alone or in conjunction with rolls to control thickness but this method is effective only at certain strip speeds.

The coating thickness is controlled in accordance with the present invention by magnetic means which sets up a magnetic field adjacent the strip. The magnetic means preferably comprises an electromagnet positioned relative to the path of movement of the strip such that the lines of magnetic flux are cut by the coating of the strip. As the strip is moved in the magnetic field, the magnetic field induces electric currents in the moving metal coating, which currents also set up a magnetic field. The resulting interaction between the field of the magnetic means and the field of the coating currents causes a force to be exerted on the current carrying coating, the force being in a direction opposite to the direction of movement of the strip. The magnetic means is located to exert the force on the coating before it has solidified, and the force, in effect, wipes the coating to produce the desired coating thickness.

Objects and advantages of the invention will become apparent from the following detailed description taken in conjunction with the accompanying figures of the drawings, in which:

FIG. 1 is an illustration of coating apparatus embodying the invention;

FIG. 2 is an enlarged fragmentary view of a portion of the apparatus shown in FIG. 1;

FIG. 3 is another view of the portion of the apparatus;

FIG. 4 is a perspective view of the portion of the apparatus;

FIGS. 5, 6 and 7 are still further enlarged views of a portion of the apparatus; and

FIG. 8 is a schematic wiring diagram of the apparatus.

While apparatus embodying the invention may be used in connection with other coating processes, it is described herein in connection with a hot dip galvanizing process. In such a process, a strip 10 of low or medium carbon cold reduced steel is fed from a supply coil 11 through treating devices 12 which prepare the strip for coating. The treating devices are similar to those shown in Pat. No. 3,322,560, and comprise an oxidizing furnace, a furnace having a reducing section, and a cooling section. At the downstream end of the treating devices 12, the strip 10 is passed over a turndown roll 13 and then moved downwardly through a snout 14 which extends below the surface level 16 of a molten metal bath 17 in a pot 15. In a galvanizing process, the metal bath 17 is of course primarily zinc. Within the metal lbath 17, the strip 10 passes around the underside of a sinker roll 18 and then extends upwardly past apparatus 19 for controlling the thickness of the metal coating. Above the apparatus 19 may be provided banks 21 of cooling sprays of air, water, Wet steam or combinations thereof, for rapidly cooling the strip 10 and the coating thereon, and the strip 10 is then passed over a guide roll 22 and out of the coating apparatus.

During the operation of the process, the strip 10 is drawn off of the supply coil 11, passed through the treating devices 12 where it is prepared for the coating oper ation, drawn downwardly through the snout 14 and into the molten metal 17 in the pot 15 and pulled upwardly through the apparatus 19 for controlling the coating thickness and through the cooling banks 21.

With reference to FIGS. 2, 3 and 4, the apparatus 19 for controlling the thickness of the metal coating on the strip comprises a pair of electromagnetic devices 26 and 27 including housings, to be described hereinafter. The devices 26 and 27 are supported by a support frame 2'8 above the level 16 of the molten bath 17. The devices are on opposite sides of, and preferably at equal distances from, the strip 10 which is pulled from the roll 18 substantially vertically upwardly out of the bath 17. The support frame 28 is mounted on a pair of vertically adjustable jacks 29 (FIG. 3) and an I-beam frame 31. The I-beam frame 31 is supported by piers 32 adjacent the pot 15, the I-beam frame 31 extending at least partially around the pot 15 and also supporting the sinker roll 18. The sinker roll 18 is rotatably mounted between a pair of spaced apart sinker roll arms 33 (FIG. 2) which have the sinker roll 18 fastened to their lower ends and extend upwardly out of the pot 15. The upper ends of the arms 33 are secured to a beam 34 which in turn is supported by the I-beam frame 31.

The support frame 28 comprises four metal bars 36a to 36d (FIG. 4) which are rigidly secured together at their ends to form a rectangle. At opposite ends of the frame 28, a plurality of structural members 37 are rigidly secured to the outsides of the bars 36b and 36d, and the jacks 29 engage the members 37 to support the frame 28.

The electromagnetic devices 26 and 27 are suspended from the frame 28 such that they may be adjusted toward or away from the strip 10. The frame 28 further includes a pair of shafts 41 and 41a which extend perpendicularly to the strip 10 and are secured to the bars 36a and 360, and the devices 26 and 27 are slidably mounted on the bars 41 and 41a by slide blocks 42 and 42a. Brackets 43 are rigidly secured to the blocks 42 and 42a and extend downwardly to support the electromagnetic devices. To adjust the positions of the blocks 42 and 42a on the shafts 41 and 41a, respectively, a pair of adjusting screws 46 and 4641 are provided adjacent and parallel to the shaft 41. The screw 46 is associated with the device 26 and is fastened to the bar 360 by a support 47 and to the block 42 for the device 26 by a nut 49. Similarly, the screw 46a is associated with the device 27 and is fastened to the bar 36a by a support 47a and to the block 42 for the device 27 by a nut 49a. The supports 47 and 47a permit rotative movement of the screws but prevent axial movement. Thus, rotative movement of the screws 46 and 46a causes the nuts 49 and 49a to travel along the length of the screws, and handwheels 48 and 48a are fastened to the screws 46 and 46a to facilitate such movement. Movement of the nuts 49 and 49a of course causes sliding movement of the blocks 42 and 42a along the shafts 41 and 41a and movement of the devices 26 and 27 toward or away from the strip 10, depending upon the direction of rotation of the screws.

Thus, the location of the devices 26 and 27 may be vertically adjusted relative to the level 16 of the molten bath 17 by the jacks 29, and the two devices 26 and 27 may be laterally adjusted toward or away from the strip 10 by the handwheels 48 and 48a.

With reference to FIGS. 4 through 7, each electromagnetic device 26 and 27 includes a housing 51 and 52, respectively, a magnetic core 53 formed of a plurality of stacked laminations, and a plurality of electrical coils or windings 54. In the present instance, the devices are constructed for use with a three phase AC power supply. The laminations forming each core 53 are punched to form pole faces 56 and are secured together by bolts 57 and bars 58. The bars 58 are positioned on opposite vertically extending sides of the laminations and the bolts are positioned through holes in the laminations and the bars to tightly secure them together. The bars 58 further include extensions 58a which extend upwardly above the cores 53 and are secured to the brackets 43 as seen in FIG.

FIG. 8 illustrates schematically the coils 54 and the power supply therefor. The power supply comprises three power input lines 61 connected to the input terminals of a continuously variable autotransformer 62 which is provided to permit adjustment of the voltage to the coils 54. The input voltage on the lines 61 may, for example, be 480 volts, three phase. The output terminals of the autotransformer 62 are connected by three power lines 64a, 64b and 64c to a power factor correcting variable capacitor bank 66 and to the coils 54 of the two electromagnetic devices 26 and 27. The power factor correcting capacitor bank 66 includes a plurality of adjustable capacitors 71 connected, for example, in a delta configuration.

The coils 54 (FIG. 8) are connected in a Y configuration, and four parallel connected windings are provided for each phase. Four windings A- l, A-4, A-12 and A-9 are connected to the power line 64a by a conductor 73; four windings B-2, B-5, B-11 and B-8 are connected to the power line 64b by a conductor 74; and four windings C-3, C-6, C-10, and -7 are connected to the power line 64c by a conductor 75. The letter S adjacent the windings in FIG. 8 indicates the start end of a winding and the letter F indicates the finish end of a winding, and it will be noted that the connections to the windings A-l, A-9, B-S, B-ll, C-3 and 0-7 are opposite the connections to the other windings, and the connections to the power supply are such that the phase sequence is from conductor 73 to conductor 74 to conductor 75.

With reference to FIGS. to 7, the windings are fastened to the core 53 such that each winding encircles two pole faces 56, and the windings are assembled on the cores such that the start end of each winding faces toward the center line of the machine. With the arrangement of the windings as shown and described, a moving magnetic field will be set up by the two devices 26 and 27 similar to the field set up by a linear polyphase induction motor. The lines of flux of the magnetic field extend substantially perpendicularly to the plane of the portion of the strip between the devices 26 and 27, and the field moves downwardly as seen in FIGS. 1 to 3. Thus, the total relative rate of movement between the field and the strip is the rate of movement of the field plus the rate of movement of the strip. Such relative movement induces currents in the metal coating, and consequently forces are exerted on the coating in a direction opposite the direction of movement of the strip similar to the force exerted by the stator of a polyphase induction motor on the rotor of the motor. The devices 26 and 27 are mounted sufiiciently close to the level 16 of the bath 17 that the coating passes between the devices before it has solidified, and consequently the force on the coating wipes the coating downwardly to produce the desired coating thickness. Each core, shown in dashed lines in FIG. 3, should be sufiiciently wide that it extends beyond the edges of the strip 10.

The thickness of the coating on each side of the strip 10 may be varied by adjusting the magnitude of the voltage applied to the windings, using the autotransformer 62. It will be obvious that as more power is delivered to the windings, the magnetic field will become stronger and the wiping force on the molten metal will increase. The thickness of the coating may also be varied by adjusting the distances between the strip 10 and the electromagnetic devices using the handwheels 48 and 48a. It will be apparent that the force on the molten metal coating will increase as the electromagnetic devices are moved closer to the strip 10. Still another way of controlling the coating thickness is by adjusting the vertical distance between the electromagnetic devices and the level 16 of the molten bath 17. Since the molten metal coating cools and becomes less fiuid as the strip is drawn from the molten bath 17, the electromagnetic field will have a greater wiping effect on the coating when it is positioned relatively close to the bath level 16.

In one construction including apparatus embodying the invention, the two electromagnetic devices 26 and 27 were positioned approximately 6 inches above the level 16 of the bath 17, and the strip speed was between 20 and 50 feet per minute. The total air gap between the two devices 26 and 27 was approximately /2 inch to 1 inch, and the strip was centered in the gap. The current in the coils was at least 320 to 365 amps. The strip was 6 inches wide and was 20 gauge stock.

In the event the strip tends to move toward one of the two devices 26 and 27, means should be provided to hold the strip centered between the devices 26 and 27. Such means may comprise devices for holding the strip taut, or edge holding slides or rollers may be used.

The housings 51 and 52 are preferably provided around the cores 53 and the coils 54 to protect them from the molten metal. The housings may be provided with means for cooling the cores and coils, and such cooling means may comprise a duct 77 at the top side of each housing, the ducts 77 being adapted to be connected to a source of cooling air. The interior of each housing may include bafiles (not shown) for directing the cooling air downwardly around the center of the core and then upwardly at the sides of the cores and out of the housing at the top side thereof.

The apparatus disclosed herein may be used with any electrically conductive coating material such as tin, terne or lead, aluminum, zinc-magnesium alloy or zinc-magnesium-aluminum alloy. The material being coated may be an electrical conductor or nonconductor.

I claim:

1. Apparatus for coating a strip with metal and controlling the thickness of the coating, comprising means for applying a coating of molten metal to at least one side of the strip, means for generating a moving magnetic field, and means for moving said strip through said magnetic field before the coating has solidified such that said coating cuts the lines of magnetic flux of said field, whereby said magnetic field induces currents-in said moving coating and said magnetic field reacts with said currents to exert a wiping force on said coating in a direction which is substantially opposite to the direction of movement of the strip.

2. Apparatus as in claim 1, wherein said magnetic field generating means comprises an electromagnet including polyphase windings wound to produce a moving magnetic field when connected to a polyphase power supply, said magnetic field moving in said opposite direction.

3. Apparatus as in claim 1, wherein said strip is moved relative to said field such that said coating is substantially normal to said lines of flux.

4. Apparatus as in claim 1, wherein said means for generating a magnetic field comprises two portions, one of said portions being on each side of the plane of said strip and said lines of flux extending between said two portions.

5. Apparatus as in claim 1, and further including means for adjusting the position of said field generating means toward or away from the plane of said strip.

6. Apparatus as in claim 1, and further including means for adjusting the position of said field generating means toward or away from said coating applying means.

7. A strip coating process, comprising the steps of coating at least one side of a strip with molten metal, generating a moving magnetic field, and while said metal is still molten moving said strip through said magnetic field such that said strip cuts the lines of magnetic flux of said field, whereby said magnetic field exerts a force on said References Cited UNITED STATES PATENTS 2,708,171 5/1955 Inglefield. 2,967,114 1/ 1961 Mayhew. 3,058,840 10/1962 Kerr et a1. 3,313,907 4/1967 Geisel et a1.

FOREIGN PATENTS 42,762 1/1967 Japan.

20 ALFRED L. LEAVI'IT, Primary Examiner J. H. NEWSOME, Assistant Examiner U.S. Cl. X.R. 

